Asymmetric carbon road rims

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biwa
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Joined: Fri Sep 21, 2018 8:39 pm

by biwa

When it comes to road rims, it seems most asymmetric rims are alloy and very few are carbon. On the other hand, there are more asymmetric carbon rims for MTB. Why? And how much real benefit is there for asymmetric road rims in general?

alcatraz
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Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

The improved lateral stiffness is nice. I noticed I can now order chinese rims asymetric without paying a premium. Usually it's only the big manufacturers that make them. :thumbup:

I wonder if there are any consequences of running asymetric. Like the rim deflects differently left/right or is much less aero on one side.

/a

by Weenie


Karvalo
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Joined: Fri Aug 10, 2018 6:40 pm

by Karvalo

Carbon road rim shapes are generally designed with aero in mind, so offset shapes would look pretty funky on any but the absolute shallowest. You do sometimes see offset spoke attachment.

jeanjacques
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Location: France

by jeanjacques

With a low height rim, assymetric rim add benefit for sure (even more with disc brake) but with a tall height rim, the spoke angle become better and the need of assymetric is less important. The spoke angle between an assymetric 21mm height rim (DT RR421 for example) and a 50mm normal one will be pretty close.

NovemberDave
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by NovemberDave

Good answers so far. You also notice that shallower rims compress more (they lose spoke tension with a tire installed), which makes the added non-drive side (or non-disc side in a front disc rim) spoke tension you get with an offset rim quite nice. The lateral stiffness shouldn't change much as you are simply redistributing bracing angle, not adding to it, but there might be some nominal increase.

It also generally gets harder to manufacture an offset spoke bed with deeper rims. It's hard to keep the nipples on the steeper spokes (drive side rear, disc side front) pointing in the right direction. This looks like the spoke holes are drilled badly, but it's really an issue with the spoke bed layup. Alloy rims don't have this issue, but we also don't see alloy rims much deeper than ~30mm.

biwa
Posts: 148
Joined: Fri Sep 21, 2018 8:39 pm

by biwa

jeanjacques wrote:
Fri Mar 01, 2019 1:29 pm
With a low height rim, assymetric rim add benefit for sure (even more with disc brake) but with a tall height rim, the spoke angle become better and the need of assymetric is less important. The spoke angle between an assymetric 21mm height rim (DT RR421 for example) and a 50mm normal one will be pretty close.
NovemberDave wrote:
Fri Mar 01, 2019 2:42 pm
Good answers so far. You also notice that shallower rims compress more (they lose spoke tension with a tire installed), which makes the added non-drive side (or non-disc side in a front disc rim) spoke tension you get with an offset rim quite nice. The lateral stiffness shouldn't change much as you are simply redistributing bracing angle, not adding to it, but there might be some nominal increase.

It also generally gets harder to manufacture an offset spoke bed with deeper rims. It's hard to keep the nipples on the steeper spokes (drive side rear, disc side front) pointing in the right direction. This looks like the spoke holes are drilled badly, but it's really an issue with the spoke bed layup. Alloy rims don't have this issue, but we also don't see alloy rims much deeper than ~30mm.
So beyond what rim height the asymmetric design starts to make less sense? 35mm? 40mm?

jeanjacques
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Location: France

by jeanjacques

Never seen a study about aerodynamic of asymmetric rim but we can say it's not the best shape, so if you are looking at 35mm rim it's already because you look at the aerodynamic gain. Asymmetric make sens for alloy wich is less stiff or MTB carbon rim which have to endure more stress.

NovemberDave
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by NovemberDave

You could easily say that asymmetric rims will have different aerodynamic properties from side to side, but so do bikes in general (because of drivetrain) and so do disc wheels in general (because of the discs).

From my perspective, the assymetric designs start to decline in relevance somewhere near your 35 to 40mm guess. This is because the compression of the rim under tire load really starts to decline there (at least in very well made rims) and because the bracing angles really start to improve somewhere around there.

We made a 45mm deep disc-specific asymmetric wheel a few years ago, and though it was quite good the manufacturing challenges introduced by the offset did not outweigh the benefits. If I was starting from scratch with that project, I would make it symmetric. The symmetric version of that rim would wind up very very similar to the rims we use now in our Cafe Racer builds.

by Weenie


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F45
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by F45

Carbon road rims with their deep dish are already plenty laterally stiff.

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